1. Technical Field of the Invention
The present invention relates generally to a power supply control system for a heater working to heat a gas sensor such as a gas concentration sensor up to a desired activation temperature which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of a specified gas component such as O2, NOx, or CO contained in exhaust emissions from the engine.
2. Background Art
Air-fuel ratio control for automotive internal combustion engines is typically accomplished using an output of a gas concentration sensor. Such a gas concentration sensor has a sensor element which includes a solid electrolyte member made of zirconia. The sensor element works to measure the concentration of a given gas component (e.g., oxygen) of exhaust gasses of the engine. An air-fuel ratio control system determines an air-fuel ratio as a function of the measured concentration of the gas component. Ensuring the accuracy of such a determination requires keeping the sensor element at a desired activation temperature. This is usually achieved using a heater embedded in the sensor element. The amount of heat generated by the heater is regulated, for example, by changing the duty cycle of a pulse signal used to switching on and off a power supply to the heater. A feedback control system is proposed which measures the resistance of the sensor element and achieves the regulation of the power supply by changing the duty cycle of the pulse signal to bring the measured resistance to agreement with a target one.
A heater power supply control system is known which supplies the power to the heater fully (i.e., the duty cycle=100%) at the startup of the engine, after which a power supply to the heater is changed in feedback control as a function of a difference between an actually measured resistance of the sensor element and a target one. The feedback control is implemented by, for example, the so-called PI control using proportional and integral gains. In the course of activation of the sensor element, that is, during a rise in temperature of the sensor element, a difference between the resistance of the sensor element and the target value is great, so that the integral gain increases gradually. This results in an excessive increase in integral gain when the resistance of the sensor element reaches the target value, which leads to overshoot of the resistance of the sensor element. The occurrence of such an overshoot results in an excessive rise in temperature of the sensor element, which may cause damage to the sensor element.
A rapid change in ambient temperature of the sensor element after the sensor element is activated completely will result in a delay in the feedback control, thus leading to overheating of the sensor element.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a heater control system for gas concentration sensors which is designed to avoid overheating of a sensor element, thereby protecting the sensor element against thermal breakage.
According to one aspect of the invention, there is provided a heater control apparatus for controlling a temperature of a heater used to heat a solid electrolyte-made sensor element of a gas concentration sensor up to a temperature at which the sensor element is activated to provide a correct gas concentration output. The heater control apparatus comprises: (a) a control circuit working to control a power supply to the heater up to a desired activation temperature; (b) a sensor element resistance determining circuit working to determine a resistance value of the sensor element of the gas concentration sensor; and (c) a heater control variable determining circuit determining a heater control variable for controlling the power supply to the heater in the control circuit based on a value of an integral term in a control function which is determined as a function of a difference between the resistance value determined by the sensor element resistance determining circuit and a target value. The heater control variable determining circuit puts a limitation on increasing of the value of the integral term until the resistance value of the sensor element reaches a preselected value in the course of activation of the sensor element. If the integral gain is increased excessively in the course of heating of the sensor element from a cold state thereof, it may cause the resistance value of the sensor element to overshoot the target value, thereby resulting in an excess increase in temperature of the sensor element, leading to thermal breakage thereof. The heater control apparatus works to limit the increasing of the value of the integral term during the activation of the sensor element for avoiding such a problem.
In the preferred mode of the invention, the heater control variable determining circuit sets the value of the integral term to zero until the resistance value of the sensor element reaches the preselected value.
The heater control variable determining circuit may determine the heater control variable only using a value of a proportional term in the control function defined in proportional plus integral control until the resistance value of the sensor element reaches the preselected value in the course of activation of the sensor element, after which the heater control variable determining circuit determines the heater control variable using both the proportional term and the integral term.
The heater control variable determining circuit may set the value of the integral term to a value defined near zero until the resistance value of the sensor element reaches the preselected value.
The heater control variable determining circuit may reset the value of the integral term when the resistance value of the sensor element reaches the preselected value during the activation of the sensor element.
The heater control variable determining circuit may work to limit a maximum value of the integral term to a preselected guard value.
The heater control variable determining circuit determines the heater control variable so as to supply power to the heater substantially fully at a given initial stage of increasing temperature of the heater and subsequently determines the heater control variable using the control function.
The heater control variable determining circuit may increase at least one of gains of the integral term and a proportional term in the control function defined in proportional plus integral control when the resistance value of the sensor element is shifted to a side on which temperature of the sensor element is increased out of a controlled range defined across the target value.
The heater control variable determining circuit may alternatively increase at least one of the gains of the integral term and the proportional term based on a temperature-resistance characteristic of the sensor element within a feedback controlled range in which the power supply to the heater is controlled as a function of the difference between the resistance value determined by the sensor element resistance determining circuit and the target value.
The gas concentration sensor may be employed to sense an exhaust gas of an automotive engine.
According to the second aspect of the invention, there is provided a heater control apparatus which comprises: (a) a control circuit working to control a power supply to a heater used to heat a solid electrolyte-made sensor element of a gas concentration sensor up to a temperature at which the sensor element is activated to provide a desired gas concentration output; (b) a sensor element resistance determining circuit working to determine a resistance value of the sensor element of the gas concentration sensor; and (c) a heater control variable determining circuit determining a heater control variable for controlling the power supply to the heater in the control circuit based on values of an integral and a proportional term of a control function used in proportional plug integral control which are each determined as a function of a difference between the resistance value determined by the sensor element resistance determining circuit and a target value. The heater control variable determining circuit increases at least one of gains of the integral and proportional terms when the resistance value of the sensor element is shifted to a side on which temperature of the sensor element is increased out of a controlled range defined across the target value.
In the preferred mode of the invention, the heater control variable determining circuit increases at least one of the gains of the integral and proportional terms based on a temperature-resistance characteristic of the sensor element within a feedback controlled range in which the power supply to the heater is controlled as a function of the difference between the resistance value determined by the sensor element resistance determining circuit and the target value.
The gas concentration sensor may be employed to sense an exhaust gas of an automotive engine.
According to the third aspect of the invention, there is provided a heater control apparatus which comprises: (a) a control circuit working to control a power supply to a heater used to heat a solid electrolyte-made sensor element of a gas concentration sensor up to a temperature at which the sensor element is activated to provide a desired gas concentration output; (b) a sensor element resistance determining circuit working to determine a resistance value of the sensor element of the gas concentration sensor; and (c) a heater control variable determining circuit determining a heater control variable for controlling the power supply to the heater in the control circuit as a function of a difference between the resistance value determined by the sensor element resistance determining circuit and a target value so as to bring the difference into agreement with the target value. The heater control variable determining circuit switches the target value between an initial value and a final value lower than the initial value during activation of the sensor element.
In the preferred mode of the invention, the heater control variable determining circuit switches the target value from the initial value to the final value when the target value has first overshot the initial value and reached the initial value again.
The heater control variable determining circuit may alternatively switch the target value from the initial value to the final value after an elapse of a preselected period of time from when the target value reaches the initial value.
The initial value may be defined within a range of the final value plus 10xcexa9.
The gas concentration sensor may be employed to sense an exhaust gas of an automotive engine.